1. Field of the Invention
The invention relates to a hydraulic drive having a differential cylinder. The invention moreover relates to a method for operating such a drive.
2. Description of the Related Art
Hydraulic drives are well known from the current state of the art. In practice it is desirable for hydraulic drives, in particular for hydraulic drives for hydraulic presses, to provide a hydraulic drive that on the one hand provides a rapid movement of a drive piston in a so-called rapid stroke or rapid movement with low force, and with which on the other hand a slower action with great force is possible in a so-called load stroke or load movement.
Various drives are known for this purpose from the current state of the art. In one drive with a so-called throttle control, a pump is driven by a motor at constant speed. The control and changeover between rapid stroke and load stroke through control of the volume flow occurs hereby via flow resistances, for example, via valves. A disadvantage of such a drive with throttle control is the low efficiency due to the occurring flow losses.
Drives having a so-called displacement control system are moreover known from the current state of the art. A drive of this type may for example have a variable speed motor that drives two pumps having opposite delivering directions. The two pumps are connected via a hydraulic cylinder in such a way that the pump takes in hydraulic oil from one piston chamber of a hydraulic cylinder and moves hydraulic oil into the other piston chamber. The changeover from rapid stroke to load stroke, or respectively the speed control of the hydraulic drive occurs through changing of the displacement volume of the pump or respectively through a change in the rotational speed of the motor. A disadvantage of such a drive with a displacement control system is that the motor must provide a higher speed for the high speed in the rapid stroke and a high maximum torque for the high force in the load stroke mode. When using a fixed displacement pump, the motor must be designed accordingly because of this high so-called peak performance, and therefore it becomes large, heavy, slow and expensive.
What is needed in the art is a hydraulic drive that can be operated in a rapid stroke and a load stroke mode such that efficiency losses can be avoided and the motor can be produced cost effectively.